"With a turnout like this, I'm beginning to feel we might win tomorrow," an upbeat Romney told a crowd of several hundred at a stop in Dunedin on Monday as he and Gingrich zipped across the state making their final appeals.

Gingrich, in turn, acknowledged that his momentum had been checked but promised not to back down. He characterized Romney as an imposter, and his team started to plot a strategy for upcoming contests.

"He can bury me for a very short amount of time with four or five or six times as much money," Gingrich said in a television interview. "In the long run, the Republican Party is not going to nominate ... a liberal Republican."

GOP officials in Florida were anticipating a big turnout, more than 2 million voters, up from 1.9 million in the Republican primary in 2008. Election officials had already received more than 338,000 absentee ballots, 37,000 more than the total early ballots cast in the GOP primary four years ago.

In the span of a volatile week, the tables have turned in this potentially pivotal primary state.

Gingrich rode a triumphant wave into Florida after a South Carolina victory nine days ago. But since then, Romney and his allies have pummeled the former House speaker on TV and on the campaign trail. Romney turned in two strong debate performances, while Gingrich faltered. Now opinion polls show the former Massachusetts governor with a comfortable lead here.

Romney and Gingrich have been the only two candidates to compete in Florida in earnest. Neither former Pennsylvania Sen. Rick Santorum nor Texas Rep. Ron. Paul campaigned much in the state, and they were elsewhere on Monday.

Clearly in command, Romney flew to stops in media markets in northern Florida and the populous swing regions of central Florida, determined to keep Gingrich from surging late.

Romney renewed attacks on his rival as an untrustworthy, Washington influence peddler at the outset of two separate appearances Monday. He claimed that Gingrich's ties to federally backed mortgage giant Freddie Mac have hurt the former speaker in a state wracked by the foreclosure crisis.

"He made $1.6 million in his company, the very institution that helped stand behind the huge housing crisis here in Florida," Romney said in Dunedin. Gingrich's consulting firm received more than $1.5 million from the federally backed mortgage giant over a period after he left Congress in 1999.

Gingrich plowed ahead, flying to stops in northern Florida starting in Jacksonville — near his home state of Georgia — before touching down in conservative Pensacola and then Tampa.

Along the way, he tried to rally conservatives by casting Romney as an imposter and himself as the true GOP stalwart. His claim to conservative principles wasn't limited to economic issues.

"No politician, no judge, no bureaucrat can come between you and God," Gingrich told an audience in Tampa. "I'm a little bit tired of being lectured about respecting every other religion on the planet."

Gingrich, who has sought to wrap himself in the mantle of Ronald Reagan, campaigned with the late president's son Michael. He was also joined by former GOP presidential candidate Herman Cain, who endorsed him Sunday.

A win by Romney would again reset the 2012 GOP race, seen early this month as his to lose, then thrown into doubt by Gingrich's come-from-behind win in South Carolina.

Romney easily won the New Hampshire primary after nearly winning the in leadoff Iowa. The South Carolina setback behind him, Romney sought to aggressively stop Gingrich, aided by a well-funded political action committee that supports him and is run by former political aides.

Together, Romney's campaign and the supportive group Restore Our Future have combined to spend $6.8 million on television ads in the final week of the Florida campaign, about three times what Gingrich and a group supporting him have spent.

Romney capped his Iowa, New Hampshire and South Carolina campaigns with upbeat spots. Nothing doing in Florida, where he was running out the clock with ads attacking Gingrich over Freddie Mac and an ethics violation in Congress.

Rick Tyler, a former top Gingrich aide now running a pro-Gingrich political action committee, showed up at Romney's kickoff event in Jacksonville on Monday, stealing a page from Romney's Florida playbook.

"I'm here to get as many cameras and microphones so I can talk about Mitt Romney's incessant failure to tell the truth," Tyler said.

Gingrich said he was confident he could narrow Romney's margin in public voter surveys, even as he and his campaign began trying to soften the blow a defeat in Florida might bring.

Gingrich aides tried to diminish the state's impact on the quest for the nomination by issuing a memo from his political director, Martin Baker.

It noted that by Wednesday morning, only 5 percent of the 2,288 national convention delegates will have been awarded.

Gingrich, who has promised to campaign through the national convention this summer, was clearly looking to regroup after Florida.

"The campaign is shifting to a new phase where opportunities are not limited to a single state," Baker wrote.

Gingrich had not announced his plans for Wednesday. Romney, who has already begun advertising in next-up Nevada ahead of the state's Feb. 4 caucuses, was traveling there Wednesday, and to Minnesota, which holds its caucuses on Feb. 7.

Associated Press writers Kasie Hunt in Dunedin, Shannon McCaffrey in Tampa and Brendan Farrington in Miami contributed to this report.